Human papillomavirus (HPV) causes thousands of cases of cancer each year, but only about half of adolescent girls in the U.S. receive even one dose of the three-dose HPV vaccine series that can prevent these cancers. Despite its demonstrated safety and effectiveness, coverage with HPV vaccine has improved little in recent years, even as other adolescent vaccines, including tetanus, diphtheria, and pertussis booster (Tdap), continue to achieve steady gains. Public health agencies, including the Centers for Disease Control and Prevention, have sought to address disappointing levels of HPV vaccination by implementing quality improvement programs for vaccine providers and promotional activities for parents and adolescents. These programs are typically implemented on a year-round basis, but much of this work may be ill-timed. Anecdotal evidence suggests that healthcare providers administer many more doses of adolescent vaccines during the summer months than they do during the rest of the year. Our pilot study in North Carolina found that providers delivered 2.02 times as many doses of HPV vaccine and 4.70 times as many doses of Tdap in August compared to December. The reasons driving this summertime peak in vaccination are unclear, but they likely involve states' policies requiring that students receive Tdap vaccine prior to school entry. The specific aims of the proposed research project are to (1) identify summer peaks in HPV and Tdap vaccination, as well as concomitant vaccination with both doses during the same visit, nationally and in U.S. states, and (2) establish the relationships among states' school entry requirements, summer peaks in vaccination, and statewide levels of HPV vaccine initiation among adolescent girls. This project will use data from a nationally-representative sample of ~45,000 adolescent girls, drawn from the National Immunization Survey-Teen from 2008 to 2012. Data analyses will employ Edwards' method (a canonical test of cyclical trends) and longitudinal structural equation modelling. Results of this study have important implications for understanding the mechanism by which state policies influence vaccination and for timing immunization quality improvement efforts to be maximally effective. For instance, campaigns promoting HPV vaccination can concentrate their activities during the summer to capitalize on the increased volume of adolescents seeking vaccines in accordance with Tdap school entry requirements. These implications may ultimately lead to increased levels of HPV vaccination and reductions in the incidence of HPV-attributable cancers, thereby improving future health outcomes in the U.S. This fellowship will build on the applicant's prior research by supporting her training in conceptual underpinnings of cancer prevention in public health, advanced statistical methodologies, and the ethical conduct of research through the mentorship of the faculty sponsor, Dr. Noel T. Brewer. These activities will support the applicant's goal of becoming a tenure-track cancer-prevention scientist specializing in geographic disparities in health behaviors.